Just WHEN is it too young to have a child? 5 year old? Maybe a 1 year old?!

One year old child found pregnant in Saudi Arabia. Doctors says it’s a unique case for this world!
Medical Science Report says : “When the mother of this child was pregnant, she had 2 fetuses inside her. But one of the fetus grew inside the other, this why this girl was born with the other fetus inside her womb.”

But to stay on topic.
Things like that happen on varying degrees. At times the other foetus is merely attached to the normally-developing one (like a parasite) or its cells are absorbed by the other (could lead to chimerism), etc.